There's a medieval text (which means the copyright is expired and it could be available online) known as The Cauldron Of Poesy that could provide hints to a sort of tradition of bardic mysticism. I read this one book that cross-referenced the Cauldron of Poesy with "The Song of Amergin" from The Book of Leinster / Lebor Gabala Erenn, and "The Invocation of Graces" from the Carmina Gadelica by Alexander Carmichael. (The Welsh bardic traditions seem to me to have been better-documented or at least more prolifically published.)
If you mean working with the fair folk, then those are usually learned directly from the fair folk.
In "The Celtic Twilight" W.B. Yeats (the same as the poet) wrote an essay about the sorcery that was done in Ireland, beginning it with how there isn't a lot of sorcery done there (and back then, when Yeats wrote it.) Then he adds this hilarious footnote about there being so much more sorcery in Ireland than he could ever have dreamed of when he first wrote that essay, but there's not nearly as much as in Scotland. I only thought it was funny because first I'm not Scottish and second in everything I've read by Yeats about the paranormal he cannot resist having at the Scots and sometimes the French and I don't know what his problem even is. So I don't know if that's reliable, but it's definitely entertaining!